This invention relates to a space-based or airborne synthetic aperture radar (SAR) system with moving target indication (MTI). In particular, this invention relates to such a radar system that employs modern space-time adaptive processing (STAP) techniques to provide subclutter visibility of slow moving targets embedded in surface clutter.
Synthetic aperture radar (SAR) systems are commonly employed on airborne and space-based platforms to provide high resolution imaging of the earth""s surface and stationary targets. SAR systems are used in a variety of remote sensing applications. Most commonly, single-channel (i.e. one antenna connected to a single receiver) systems are employed. However, dual-channel systems (i.e. two antennas connected to two receivers) are of more recent interest and are used in applications requiring cross-track interferometry (which facilitates height determination) and polarimetric information (useful for identification of image features). SAR systems employ a variety of SAR signal processing techniques known and used by those skilled in the art. A general treatment of such techniques can be found, for example, in Curlander and McDonough, Synthetic Aperture Radar Systems and Signal Processing, Wiley, 1991. These techniques generally assume the objects being imaged are stationary. High resolution images are formed in the range and cross-range (also called azimuth) dimensions of the image using high-bandwidth waveforms, and long dwells which have the effect of creating a large synthetic aperture. Range compression techniques are generally employed to compress the coded waveforms, thereby producing the desired range resolution. Over the duration of the dwell, waveform pulses are transmitted and received coherently and subsequently compressed in azimuth to produce the desired azimuth resolution. Over the duration of the dwell, the moving platform carrying the antenna traverses a large distance relative to the real antenna aperture dimension, thereby forming a synthetic aperture. The resulting fine azimuth resolution is intuitively related to the dimensions of this synthetic aperture analogous to the (coarse) azimuth resolution associated with a real aperture. Depending on the application, but generally speaking, individual scatterers being imaged can walk in range and azimuth due to the motion of the platform. As a result, range and azimuth correction techniques are often required in addition to the range and azimuth compression steps. Other platform motion compensation steps may be additionally required, depending on the system and the application. These steps can either be done in the radar hardware (by adjusting oscillators and sampling times) or in the digital processing.
It is well understood by those skilled in the art that imaging moving targets using conventional SAR""s has many problems making performance generally unacceptable for a variety of applications. Target motion can result in significant degradations in signal strength and image resolution, making detection of moving targets difficult or impossible. Furthermore, moving targets are displaced from their true locations in images, requiring additional estimation and correction techniques to be employed. Although there have been approaches suggested to provide a single-channel SAR with moving target detection and imaging (see for example Freeman and Currie, xe2x80x9cSynthetic Aperture Radar Images of Moving Targetsxe2x80x9d, GEC Journal of Research, Vol. 5, No.2, 1987), these approaches are applicable to a limited number of systems (usually airborne systems where a PRF several times larger than the clutter bandwidth may be employed) and applications (targets with sufficient radial velocity relative to the clutter bandwidth so as to move clear of the clutter).
To reliably detect slow and fast moving targets in clutter-limited scenes, moving target indication (MTI) techniques are generally employed, which combine signals from multiple (two or more) channels to suppress unwanted clutter and provide improved moving target detection and parameter estimation. When the radar is not moving (e.g. for ground-based systems), moving targets are easily detected by the simple use of pulse-canceler circuits. Only moving targets will have a Doppler shift away from DC (i.e. zero frequency) which allows them to escape cancellation by the pulse canceler. In airborne radar systems, returns from stationary objects (e.g. the ground or stationary targets) have non-DC Doppler shifts due to the motion of the platform. Those skilled in the art recognize that the mainbeam ground returns span a large clutter Doppler bandwidth that is proportional to the platform velocity and the azimuth beamwidth (resolution) of the antenna. The clutter bandwidth commonly spans the entire signal spectrum, thereby covering moving target returns. As a result, it is necessary to provide subclutter visibility in order to detect small moving targets. Many multi-channel systems and associated signal processing techniques have been developed to provide moving target detection and estimation for airborne radars. See for example iSkoinik, Radar Handbook, Second Edition, Chapter 16, McGraw-Hill Inc., 1990. These techniques are often referred to as AMTI (for airborne moving target indication) techniques. Moving targets can be both airborne targets and ground-based targets. Ground-based targets such as vehicles (including tanks and jeeps) travel slower than airborne targets such as aircraft and missiles. However, relative to the clutter, ground targets and air targets can both move slowly .
The large majority of moving target indication (MTI) systems and techniques have been developed for airborne radar systems. This is evident from the numerous open literature and patent literature publications. Furthermore, there are numerous airborne MTI radars in use today. For systems and techniques designed for operation in an air-to-air or air-to-ground mode, the term AMTI (for airborne moving target indication) has been generally used in the literature. Some airborne literature uses the term GMTI (for ground moving target indication), however, specifically for the air-to-ground mode. Description of space-based MTI radar systems and techniques is virtually nonexistent in the patent literature, and quite limited in the open literature. See for example Nohara et al., xe2x80x9cA Radar Signal Processor for Space-Based Radarxe2x80x9d, 1993 IEEE National Radar Conference, April 1993, and Nohara, xe2x80x9cDesign of a Space-Based Radar Signal Processorxe2x80x9d, IEEE Trans. Vol. AES-34, No.2, April 1998. Furthermore, there are no known space-based MTI radars in use today. For space-based radars which operate in a space-to-air or space-to-ground mode, the term AMTI has been used for the former mode, and AMTI or GMTI for the latter mode, following airborne systems. Recognizing that for the most part, the same body of signal processing techniques and system elements are employed in air-to-air, air-to-ground, space-to-air and space-to-ground modes, the term MTI is deliberately used herein so as not to limit the scope of the invention to a specific space-to-air, space-to-ground, air-to-air or air-to-ground system or mode of operation, as well as to avoid confusion.
MTI techniques combine multiple channels to cancel or attenuate unwanted clutter. Selected multiple channels are combined by multiplying each selected channel by an appropriate weight and adding the resulting weighted channels together to produce the output, clutter-suppressed channel. If the weights used are fixed (i.e. pre-determined), then the signal processing techniques are referred to as xe2x80x9cfixedxe2x80x9d MTI techniques. On the other hand, if the weights are computed adaptively (i.e. they depend on the received data) as for example in Brennan et al., xe2x80x9cAdaptive Arrays in Airborne MTI Radarxe2x80x9d, IEEE Trans. Vol. AP-24, September 1976, then the term xe2x80x9cadaptivexe2x80x9d MTI techniques is used. Adaptive MTI techniques have the potential to provide greater clutter suppression than fixed MTI techniques; but have higher processing costs. Fixed or adaptive MTI systems refer to systems employing fixed MTI or adaptive MTI techniques, respectively.
From the above discussion, one can deduce that integrating SAR and MTI techniques into a single system has the potential of providing reliable detection, estimation and imaging functions for both stationary targets and moving targets. Several airborne SAR-MTI systems and techniques have been proposed and a few systems are operational. Raney alludes to a simple two channel SAR that employs a simple two-pulse MTI canceler to reduce clutter (see Raney, xe2x80x9cSynthetic Aperture Imaging Radar and Moving Targetsxe2x80x9d, IEEE Trans. Vol. AES-7, No. 3, May 1971). In U.S. Pat. No. 3,993,994, Goggins describes a two-channel airborne SAR-MTI system where the DPCA (displaced phase center antenna) condition may or may not be satisfied (i.e. a trailing antenna""s phase center occupies the spatial position of its adjacent, leading antenna""s phase center an integral number of pulse repetition intervals later in time), and where SAR processing is performed independently on both channels followed by a simple, adaptive, two pulse canceler. A single adaptive weight is computed from the received data and applied to the trailing channel and updated over time in an attempt to compensate for adverse effects from unknown, uncalibrated, time varying parameters. In U.S. Pat. No. 5,122,803, Stan and Alexander describe an N-channel, sidelooking, airborne moving target imaging system that incorporates both SAR processing and MTI processing. The N-channels are arranged along the platform velocity vector so that a DPCA condition is satisfied. The effect is to spatially arrest the apertures for successive instants in time. SAR processing is performed on each of the N channels, followed by Doppler processing (which is similar to fixed-weight MTI processing in that the stationary clutter can be canceled by ignoring the DC Doppler bin) across the N channels. In the more recent U.S. Pat. No. 5,818,383, Stockburger et al. provide a different real-time, airborne, air-to-ground SAR-MTI solution which forms the basis of the JSTARS system. This system is designed to detect and locate ground moving targets in a manner quite similar to that described in Nohara et al., xe2x80x9cA Radar Signal Processor for Space-Based Radarxe2x80x9d, 1993 IEEE National Radar Conference, April 1993, and Nohara, xe2x80x9cDesign of a Space-Based Radar Signal Processorxe2x80x9d, IEEE Trans. Vol. AES-34, No. 2, April 1998; except Stockburger et al. use fixed MTI rather than adaptive MTI. Once detected and located, the moving targets are then imaged and overlaid on SAR imagery of the scene containing the targets.
Stockburger et al. claim their approach is better suited for real-time implementation than Stan and Alexanders"" approach since Stockburger et al. perform full-scene (i.e. for the full set of velocity and range cells) SAR processing only on a single channel, whereas Stan and Alexander do so on all N channels. Furthermore, Stockburger et al. claim their approach is better suited to accelerating targets, since they detect and estimate target parameters on subdwells, and only then form target images using the full dwell. Stockburger et al.""s approach suffers from poorer detection performance, however, when the integration gain that could be provided from the whole dwell is needed for reliable detection. This situation can arise for smaller targets, for slower targets which are more attenuated by the MTI filters, and for cases which are power or noise limited, as can occur in space-to-ground and space-to-air applications, where power is a premium, and where very large two-way propagation losses are standard. The approaches of Goggins, Stan and Alexander, and Stockburger et al. all suffer from channel-to-channel differences. Those skilled in the art will appreciate that uncompensated differences between the antennas and receivers can be the limiting factors in clutter suppression performance. For example, differences in antenna patterns due to mainbeam pattern differences and random sidelobe behavior must be corrected for improved performance. The fixed MTI filters used by Stockburger et al. and Stan and Alexander will not provided optimum performance in many cases. Furthermore, although an attempt was made by Goggins to use an adaptive weight, only a single adaptive weight is applied to correct all unknown differences between the two channels. This kind of adaptive weight is only capable of correcting for an overall channel gain difference and phase difference, and does nothing to compensate for antenna pattern differences, for example, which are a function of azimuth (or Doppler).
There are no known space-based SAR-MTI systems that are operational today, although space-based SAR-MTI is the subject of current research and development. Canada expects to have the world""s first, space-based, experimental, SAR-MTI radar with the launch of Radasat 2 planned for 2002.
An object of the present invention is to provide a SAR-MTI system suitable for space-based applications.
Another object of the present invention is to provide a SAR-MTI radar system having the ability to produce high-resolution SAR images of the earth""s surface with detections from moving targets overlaid.
A further object of the present invention is to provide a SAR-MTI radar system capable of detecting, estimating parameters of, and imaging slow-moving targets in strong clutter.
Another object of the present invention is to provide a SAR-MTI radar system capable of detecting, estimating parameters of, and imaging targets free from clutter.
Yet another object of the present invention is to provide a SAR-MTI radar system that can simultaneously operate in SAR mode and MTI mode in parallel, using the same acquired data.
Still another object of the present invention is to provide a SAR-MTI radar system with improved detection performance and parameter estimation performance for small targets and slow targets.
Another object of the present invention is to provide a SAR-MTI radar system which uses adaptive MTI techniques to compensate for Doppler-dependent channel mismatches; and to provide improved clutter suppression performance.
An additional object of the present invention is to provide a SAR-MTI radar system that is robust or resistant to mainbeam jamming.
Yet another object of the present invention is to provide a SAR-MTI radar system with more flexible antenna/waveform design configurations.
A further object of the present invention is to provide a SAR-MTI radar system that has the SAR and MTI functions integrated so as to provide improved performance while reducing numerical computations.
A final object of the present invention is to provide a SAR-MTI radar system that is capable of real-time implementation.
In accordance with the present invention, a SAR-MTI radar system suitable for space-based applications includes a first plurality of transmitting antennas, and a second plurality of receiving antennas operatively connected to a set of coherent receivers which gather and digitize the RF signals collected by the receiving antennas. The digitized signals from each receiver channel are expanded to create a set of pseudochannels by passing each receiver""s output signal through a delay network which provides a plurality of temporal taps, where each tap provides a different delayed version of the original signal. The pseudo-digitized signals are each processed using SAR processing techniques to create high-resolution SAR imagery of the stationary scene which may contain stationary targets. Detection and parameter estimation functions automatically detect and locate stationary targets from the scene imagery. The same pseudo-digitized signals are processed in parallel to detect, locate and image moving targets. Adaptive MTI processing techniques are employed to both compensate for unknown channel-to-channel mismatches and provide optimum clutter suppression, thereby improving the performance of target detection and parameter estimation. Separate detection and estimation functions are employed to automatically detect and locate moving targets. The moving target detections can be indicated on the SAR stationary scene imagery; or separate SAR moving scene imagery can be created for the moving targets.
In the present invention, the first plurality of transmitting antennas and the second plurality of receiving antennas can take many forms. In one form referred to herein as sequential antenna operation, the transmit antennas and the receive antennas are the same. A set of N antennas is provided. In a sequential fashion, the leading antenna transmits a pulse and receives the pulse echo, following by the next leading antenna which transmits a pulse and receives its echo, and so on, until all N antennas have operated. The cycle then continues again starting with the leading antenna. The sequential antenna form has the disadvantage of reducing the effective pulse repetition frequency (PRF) by a factor of N. In a preferred form referred to herein as simultaneous antenna operation, a single transmitting antenna is used to transmit all pulses, along with a set of receive antennas which simultaneously receive the radar echos resulting from the transmitted pulses. The antennas themselves could be separate, independently provided apertures, or they could be subapertures formed from one or more main apertures. Without loss of generality, in the sequel, we will assume the preferred, simultaneous antenna operation form unless otherwise indicated.
For space-based MTI applications, one skilled in the art can anticipate that improved clutter cancellation performance will be needed. Orbiting platforms in LEO orbits travel about 7 km/s, or an order of magnitude faster than most airborne platforms. As a result, the platform-induced clutter bandwidth that a radar designer needs to deal with can be an order of magnitude larger. The large field of view available to space-based radars and the potentially large surveillance footprints used to maintain high search rates mean that nonhomogeneous clutter is likely.
The present invention uses space-time adaptive processing (STAP) techniques in its adaptive MTI processing. The present invention is further unique in that it supports a whole taxonomy of STAP techniques which can be selected or tailored to the mission at hand. The spatial dimension is created by selecting signals from pseudochannels originating from different receivers but from the same temporal taps. The temporal dimension is created by selecting signals from pseudochannels originating from the same receiver but from a series of delays or taps. Space-time systems are formed by appropriately grouping the pseudochannel signals into multiple groups or systems. Adaptive MTI processing is used to suppress clutter for each system. Those skilled in the art will appreciate the improvement in clutter suppression that is possible when STAP techniques are employed, as compared to fixed-MTI systems. Furthermore, since the number of pseudochannels is in general larger than the number of actual receiver channels, larger space-time systems can be created with the present invention, making a larger number of adaptive degrees of freedom (ADOF) available. A larger number of ADOFs has many uses, one of which is to provide steeper and better clutter suppression filters that result in less attenuation for slow-moving targets that appear close to the filter""s cut-off frequency. Hence, the minimum detectible velocity can be improved. In U.S. Pat. No. 3,993,994, Goggins provides only a single ADOF with no means to increase the number of ADOFs. In U.S. Pat. No. 5,818,383, Stockburger et al. do not use any ADOFs and employ only fixed MTI techniques.
A feature of the present invention is its ability to provide good MTI performance for situations when the phase centers of the receiving antennas are not aligned parallel to the platform velocity vector. In U.S. Pat. Nos. 3,993,994, 5,122,803, and 5,818,383, the phase centers are assumed to lie along the platform velocity vector; performance degradations result if this assumption is not maintained. In airborne systems, the phase center axis can be misaligned from the platform velocity vector as a result of wind or turbulence, or by design (for example, when the antennas are part of a rotating system). In space-based radars, a unique situation arises due to the impact that the earth""s rotation has on ground scatterers. For orbiting platforms, the earth""s rotation causes a Doppler spread to impart on radar echos from mainbeam ground scatterers. This Doppler spread can be quite substantial and must be compensated, as it has the same effect as the platform-induced clutter Doppler bandwidth. One way to compensate for the effects induced by the earth""s rotation is to yaw the antenna axis a sufficient amount to cause the zero Doppler centroid to shift to broadside. In this case, the receive antennas are misaligned from the platform velocity vector by design. Indeed, one could derive other reasons for designing receive antenna phase centers that are not restricted to lie on an axis parallel to the platform velocity vector. For example, having the phase center axis pitched relative to the velocity vector can improve DPCA cancellation for squinted looks. The availability of a user-specified number of ADOFs and the use of adaptive MTI techniques make the present invention""s performance robust to such conditions and designs.
The availability of extra ADOFs afforded by the present invention can be used to provide mainbeam jamming suppression in addition to clutter suppression. For space-based MTI applications, the footprint can be very large increasing the odds of encountering mainbeam jamming. Systems which employ fixed MTI processing will not be robust to jamming. Systems employing at least two spatial ADOFs can use one ADOF for clutter suppression, and the second to cancel a mainbeam jammer. Systems with additional ADOFs have the ability to suppress additional mainbeam jammers.
Another feature of the present invention is its robustness in MTI performance when the receiving antenna phase centers do not satisfy the DPCA condition. The systems described in U.S. Pat. Nos. 3,993,994, 5,122,803, and 5,818,383 require the DPCA condition to be satisfied if acceptable MTI performance is to be maintained. For a fixed antenna design with fixed phase center spacing, airborne systems such as those in the aforementioned patents would have to adjust the waveform PRF to maintain the DPCA condition when the velocity of the aircraft changes from the design velocity. It is desirable in most radar systems to have the flexibility to optimize the PRF for target detection or image formation, rather than for clutter suppression. The use of adaptive MTI techniques in the present invention allows one to decouple the PRF from the platform velocity by implicitly interpolating a DPCA solution adaptively from the multi-channel signal data. In space-based applications, the platform velocity is highly stable; as a result, the PRF can be optimized for SAR imaging or for a desired MTI response for moving target detection.
The adaptive MTI techniques used in the present invention are generally applied in the frequency domain. That is, adaptive clutter suppression weights are independently computed for each Doppler bin. Since Doppler is related to azimuth, this approach compensates for mismatches in the mainbeam and sidelobe antenna patterns of the receive antennas as a function of azimuth, thereby improving clutter suppression. Depending on the extent of the processed scene, a single adaptive weight could be computed for each Doppler bin to be used for all range bins, or the weights could be recomputed in a block fashion for groups of range bins. These adaptive MTI approaches also optimize clutter cancellation performance in the presence of nonhomogeneous clutter.
A unique feature of the present invention is that the SAR-MTI radar signal processing architecture can be tailored by the system operator to the mission at hand. This tailoring begins by specifying the number of space-time groups to be used, the type of STAP algorithm that will be employed in the adaptive MTI processing (a discussion of some important STAP algorithms supported is provided in the sequel), and the number and type (spatial and/or temporal) of [pseudochannels required. SAR processing using conventional SAR algorithms is performed on each of the specified pseudochannels. The operator can specify which of the SAR-processed pseudochannels will be combined and used for SAR stationary scene image. In the simplest case, one of the SAR-processed pseudochannels can be used directly as the stationary scene image. Automated detection and estimation functions can then be applied to the stationary scene image to detect and locate stationary targets. In parallel, the SAR-processed pseudochannels are also grouped into the specified space-time systems and adaptive MTI processing is performed on each system to suppress clutter and jamming. The output signals from the available systems are then used by moving target detection and estimation functions. One or more of the MTI output signals can be combined for detection and/or estimation. Conventional constant false alarm rate (CFAR) detectors known to those skilled in the art are generally used to detect moving targets. CFAR detectors work well for both clutter-limited and noise-limited conditions. The estimator functions employed can be conventional maximum likelihood (ML) estimators, or empirically derived estimators. Two special cases supported by the present invention are SAR-only or MTI-only modes. For SAR-only modes, the parallel MTI path in the radar signal processor (RSP) is disabled. For MTI-only modes, the SAR stationary scene image and detection and estimation functions are bypassed.
A preferred embodiment suitable for space-based applications (and airborne as well) has two receive antennas. The DPCA condition is approximately satisfied, which facilitates clutter cancellation. Additional ADOFs are created by providing up to three temporal taps in the delay network used on each receiver channel, creating between 2 to 6 pseudochannels. SAR processing is performed on the pseudochannels, and one of the SAR-processed pseudochannels is used as the stationary scene image. Detection and localization of stationary targets may follow. A single space-time system is formed from the available SAR-processed pseudochannels in accordance with a selected STAP algorithm. The STAP algorithm can take the form of a simple adaptive DPCA algorithm (in this case, only two pseudochannels are formed using a single delay tap from each receiver channel) employing a single ADOF, to more advanced STAP algorithms which utilize four or six pseudochannels (i.e. by processing the signals from two or three delay taps, for each receiver). The output signal from the space-time system represents an image of moving targets. Detection and estimation functions may be performed on this signal to detect and locate targets. These detections can then be overlaid on the SAR stationary scene image.
In U.S. Pat. No. 5,818,383 by Stockburger et al., SAR imaging of moving targets is only performed in the vicinity of detected moving targets. This is achieved by doing MTI over shorter subdwells and detecting and estimating target locations for each subdwell. A target track is developed for each target by filtering the target estimates obtained from the set of subdwells which span the full SAR dwell. The target tracks are then used along with the full-dwell signal from one receiver channel to create SAR imagery for each of the detected moving targets. While this approach is a special case of the present invention where the conventional SAR function is replaced by conventional pulse Doppler processing over the set of subdwells, the present invention in its general form has two advantages over the system proposed by Stockburger et al. First, by doing SAR processing on each of the pseudochannels before MTI processing, moving target imagery is formed in batch for the entire scene; not simply at locations where moving targets are detected. This is analogous to creating for a radar operator a full plan position indication (PPI) display, rather than a synthetic PPI display containing only the binary detections. Radar operators can usually interpret additional useful information from the full PPI display as compared to the synthetic PPI display. In the same way, the SAR moving target scene imagery created by the present invention is a useful output of the system. Second, the detection performance for very slow or small moving targets is better for the present invention because the coherent integration gain provided by SAR processing (i.e. use the full dwell) precedes detection. As a result, detection is performed for a target signal with higher signal to interference plus noise ratio (SINR). While it is realized that the full coherent integration gain can only result for nonaccelerating targets, this situation is true or approximately true in many applications. In applications where shorter SAR dwells are used (i.e. on the order of one second rather than up to 10 seconds as described by Stockburger et al.), the nonaccelerating target assumption can be quite valid. Most existing space-based SAR systems fall into this category.
A final feature of the present invention is its ability to be implemented in real-time for many applications, if necessary. Although the computational requirements can be quite demanding especially when multiple ADOFs are employed, the exponential improvements in computing power and the ability to partition the problem across multiple processors make real-time implementation possible on customized computing hardware. In U.S. Pat. No. 5,818,383, Stockburger et al. claim that providing moving target imagery for the entire scene does not allow for real-time implementation. While this may have been the case for JSTARS on the selected computing hardware, it is not the case in general, especially with today""s improvements in computing technology (and future improvements still expected). Stockburger et al. considered very long dwells on the order of 10 seconds (with 1 ft. resolutions). When shorter dwells and lower resolutions are employed as would be the case for many applications (especially surveillance applications), the computational requirements can often be reduced. In other applications, processing is done on the ground where larger and faster computers can be used for real-time implementation. For example, most space-based SAR systems do their radar signal processing on the ground. Finally, in many applications, real-time processing is not even a requirement. As a result, RSPs employing more sophisticated and computationally demanding processing such as in the present invention are practical today.
Finally, a preferred embodiment of the present invention facilitates real-time implementation by a clever and innovative integration of the SAR and MTI functions. The SAR function is broken up into a coarse Doppler stage followed by a fine Doppler stage. The MTI operation is moved forward within the SAR function, immediately after the coarse Doppler stage, so that the number of pseudochannels is reduced by the MTI operation before completing the fine Doppler stage. This reduction in channel count before proceeding with the computationally intensive fine Doppler stage results in significant reductions in computational costs. Furthermore, improvements in the resulting moving target SAR imagery are possible by estimating target dynamics after the coarse Doppler stage, and exploiting this knowledge in the fine Doppler stage, thereby providing a means for autofocusing.
In the sequel, more detailed descriptions of preferred embodiments of the present invention are presented, which bring to light other objects, features and advantages of the present invention. The drawings employed and the ensuing discussions are not intended to limit the scope of the present invention, but rather to provide insight into certain aspects of the present invention.